This invention relates to a movement compensation predictive encoder for a moving picture signal and, more particularly, to a movement compensation interframe predictive encoder of the type described.
A movement compensation predictive encoder is already known. By way of example, one is disclosed in U.S. Pat. No. 4,460,923 issued to Akira Hirano et al and assigned to the present assignee. Such a movement compensation predictive encoder is used in digitally dealing with a moving picture signal either for delivery to a transmission channel or for storage on a recording medium. The moving picture signal is typically a television signal and comprises successive frames. When reproduced on a viewing screen, each frame shows an instantaneous picture. For each instantaneous picture, picture elements are represented by signal elements of the moving picture signal and are successively arranged along a scanning line with the scanning line swept substantially orthogonally to the line. As the case may be, the signal elements will be referred to herein as picture elements. Merely for brevity of description, it will be assumed throughout the following that the moving picture signal is delivered to a transmission channel as a movement compensation (interframe) predictive encoded signal rather than stored on a recording medium.
For movement compensation predictive encoding of a moving picture signal, a vector detector is used in detecting movement or motion vectors from the moving picture signal to produce a movement vector signal which successively indicates the movement vectors. In the manner which will later be described, each frame is divided on detecting the movement vectors into a predetermined number of blocks of picture elements. For example, a block consists of eight scanning lines and sixteen picture elements along each scanning line. For each block, the movement vector indicates a displacement from a previous frame to a current frame. Supplied with the moving picture signal and the vector signal, a predictive encoding circuit calculates a prediction error with movement compensation to produce a movement compensated prediction error signal consecutively indicative of such movement compensated prediction errors. Each movement compensated prediction error corresponds to a difference between the picture elements of the respective blocks which are related in the previous and the current frames by each movement vector. After quantized into a quantized error signal, the prediction error signal is fed to the transmission channel as the movement compensation predictive encoded signal. Before delivery to the transmission channel, codes of the movement compensation predictive encoded signal are usually subjected to variable length encoding.
On generating a moving picture signal, use is often made of a video effect technique such that a plurality of elementary pictures are reduced in size into reduced pictures and then edited in various manners into an instantaneous composite picture with each reduced picture positioned at a small region of the composite picture. Each region may consist of one or a plurality of blocks. Such composite pictures are represented by composite moving picture signal. Before reduced, each elementary picture is represented by an elementary moving picture signal. Each reduced picture may have no relation to other reduced pictures. Some regions may therefore monotonously move in successive composite pictures. 0thers may be subjected to considerable variations or changes. It is not seldom in this manner that the composite moving picture signal has statistical characteristics which vary from region to region in each composite picture.
Attention will be directed to that part of an instantaneous picture which is in each block and will now be called a block picture. The block picture may be a part of a composite picture. Alternatively, the block picture may be a part of a picture of a single varying scene. In any event, an interframe correlation is reduced to a very small value, if not entirely lost, when a wide variation takes place between the block pictures of the previous and the current frames. For a picture of a single scene, such a wide variation appears either when a television camera is switched to another camera to change the scene to another scene or when the television camera is panned.
When a wide variation occurs between two block pictures, the movement vector signal indicates a movement vector which does not converge to a finite vector but diverges. In other words, the movement vector becomes an erroneous movement vector which does not comply with an actual movement of the block between the previous and the current frames. When such an erroneous movement vector is used, it is impossible to compensate for the movement. An objectionable increase results in an amount of information carried by the movement compensation predictive encoded signal. This means a reduced encoding efficiency and results in a reduced rate of band compression. Furthermore, the erroneous movement vector degrades the quality of reproduced pictures.
In the manner described above, a wide variation occurs between two block pictures not only on panning of a television camera but also upon occurrence of a scene change from a previous scene to a current scene. Whenever a scene change takes place, the amount of information becomes very excessive. It has, however, been impossible to correctly discriminate the scene change from the panning of a television camera or a wide variation which results from a very quick movement of a picture portion.